(1) Field of the Invention
The present invention relates to a sonar system which utilizes a gaseous cavity to eliminate hydrodynamic noise associated with turbulent boundary layers and turbulent wakes. More specifically the present invention relates to a sonar system that can be utilized on a hydrofoil marine vessel.
(2) Description of the Prior Art
Marine operators would like to operate high-frequency sonar systems from high speed surface craft. These craft can operate at speeds exceeding 30 knots, but they produce bubbly wakes (high frequency noise source), have noisy propulsors, and have noisy appendages. Sonar systems towed in or operating near their wakes are adversely affected by this generation of background noise, thus limiting their effective detection range.
A similar problem exists for very fast transport ships. Some futuristic concepts have been proposed which operate at speeds up to and exceeding 100 knots. The ability of these ships to maneuver at very high speeds is limited. Consequently, the ability to detect obstacles at significant range increases the ability of these craft to avoid collisions with marine mammals, mines, and debris.
Sonar systems towed at very high speeds are affected by several noise sources which may be controllable. The turbulent flow of water over the streamlined fairing of an array generates pressure fluctuations on the fairing. Both turbulent boundary layers and turbulent wakes contribute to this type of structural excitation. The pressure fluctuations can be experienced directly on the array when the flow over the array is turbulent, or indirectly as pressure fluctuations away from the sensor face are transmitted through the structure to the array. Fluctuating cavitation bubbles and collapsing vapor bubbles can also produce large structural excitations. The best way to minimize these types of hydrodynamic noise is to maintain laminar flow over the array face and to physically isolate the array face from portions of the structure experiencing large pressure fluctuations.
The vessel propulsion system produces a large amount of noise. Components of this noise include blade tonals, cavitation bubbles, and entrained air which produce noise that can propagate through the environment to the array. Similarly, breaking bow waves, hull slapping, ship machinery noise, and other ship related noise sources can reach the array through the environment. Isolating the array from these sources by eliminating the direct acoustic path between the source and the array would greatly improve the array performance.
A mechanical path from the noise source through the array supporting structure can create another acoustic problem for the array. However, mechanical isolation techniques are advanced and can minimize these effects.